Injectors of the type to which the present invention is directed (such injectors sometimes being referred to as "pressure infusion devices") are used for carrying out long-term infusions of a liquid into a patient. They are suitable for actuating a syringe slowly and continuously, so that the contents of the syringe are supplied to a patient connected to the outlet of the syringe in a uniform manner over an extended period of time (e.g., 24 hours). The injectors typically, therefore, supply minute quantities of liquid at a rate, e.g., of 0.06 to 6 ml/h with a high degree of uniformity of the rate of supply over the time of operation and with the velocity deviation being generally limited to about 1% maximum.
Injectors are known wherein the syringe is squeezed by means of a rack-and-pinion drive mechanism driven by a synchronous motor with reversing gear. The front end of the syringe cylinder is retained in a holder. A second holder, connected with the piston rod of the syringe, is slowly moved in the direction of the first holder.
More recent injectors are equipped with a spindle drive instead of a rack. This spindle drive is driven by a step motor or by a direct current motor via a transmission. The spindle drive can be disengaged through a release device to enable presetting the piston rod by hand. This, however, involves the following problems: When using the injector, the syringe is first filled from a bottle containing medication using a cannula. Then the cannula is removed, and an empty tube is taken from a sterile package and connected to the syringe outlet. For reasons of sterility, cleanliness, and economy, it is desirable to fill the tube exactly up to its free end before a cannula or catheter is connected to it. In known injectors, this filling of the tube is effected by advancing the slide with the release device actuated until the liquid just reaches the front end of the tube. When actuation of the release device is terminated, either an engagement with the graduations or screw-thread of the spindle or rack occurs or the engagement does not occur. If after actuation of the release device has terminated, engagement of the spindle or rack occurs, a jerking motion forward or backward may occur at the slide which results in air entering the tube or liquid being discharged from the tube. On the other hand, if engagement does not occur when actuation of the release device is terminated, the engagement will occur later with a certain delay, namely after the spindle or rack has moved sufficiently for engagement to be possible. As an example, with a rate of feed of 1 ml/h, a piston path of 80 mm in 50 hours, and a speed at the output end of 1.6 mm/h, and assuming that the spindle pitch is 1 mm, it may take as long as 40 minutes before engagement occurs. During this time, injection does not take place even though the patient is hooked up to the injector. This may lead to thrombus formation and finally embolism. For this reason, a forcible engagement is usually provided for.
If the engagement is brought about forcibly after the tube has been connected to the patient, there are two undesirable and medically risky effects. The engagement leads either to a brief reversal of the direction of flow as a result of which blood is sucked from the patient causing danger of thrombus formation and embolism, or else there is a concentrated surge of medication to the patient.
Another disadvantage of the known injectors is that when the release device is actuated under load, the initial tensions are relieved suddenly.